The invention relates to removing or scavenging hydrogen sulfide (H2S) and/or mercaptans from carbon dioxide (CO2), and most particularly relates, in one non-limiting embodiment, to removing or scavenging H2S from liquid and/or super-critical CO2.
Carbon dioxide is widely used in secondary and tertiary recovery of oil and natural gas in oil field applications. Prior to injection, CO2 is processed and purified in processing plants and compressed to about 1800 to 2000 psig (about 12,000 to 14,000 kPa) for distribution and injection. To comply with some governmental regulations, the H2S concentration of CO2 to field injection points is required to be less than 100 ppm.
Some of the plants processing and purifying CO2 are operating at their maximum capacity for handling CO2 and effectively removing H2S to maintain H2S concentrations at less than 100 ppm. The primary limitation of some plants is their capacity to remove H2S to an outlet concentration of less than 100 ppm. Operators of such facilities may experience a peak in gas production resulting in an increased requirement in capacity to handle the volume of CO2 produced. The cost for making mechanical changes to these existing plants may require several million dollars and take a year or more. A chemical treatment would be a desirable alternative to making such a capital investment. Under these conditions, such plants cannot handle the increased production. Hence, the additional volume of CO2 cannot be processed and H2S cannot be removed from the CO2 thus preventing the use of the additional CO2 for further recovery of hydrocarbons.
It is known to use triazines to selectively remove H2S from natural gas in the presence of CO2. Tauseef Salma in xe2x80x9cEffect of Carbon Dioxide on Hydrogen Sulfide Scavenging,xe2x80x9d SPE 59765, 2000 SPE Permian Basin oil and Gas Recovery Conference, Midland, Tex. 21-23 March 2000, studied the effect of CO2 on the H2S scavenging capacity of triazine-based scavengers for removing H2S from natural gas. She concluded that CO2 has very little effect on the H2S scavenging performance of a triazine based H2S scavenger.
U.S. Pat. No. 4,978,512 describes methods for selectively reducing the levels of hydrogen sulfide and organic sulfides from gaseous and/or liquid hydrocarbon streams, particularly natural gas streams, comprising contacting the streams with a composition comprising the reaction product of (i) a lower alkanolamine with (ii) a lower aldehyde. The reaction product of formaldehyde with monoethanolamine is 1,3,5-tri-(2-hydroxyethyl)-hexahydro-S-triazine.
Thus, it would be desirable if a method for removing or scavenging H2S from carbon dioxide in liquid and/or supercritical form could be discovered which involved simply adding a chemical scavenger to the carbon dioxide.
An object of the invention is to provide a method for scavenging or removing H2S from liquid and/or supercritical CO2.
An additional object of the invention includes providing a method for scavenging or removing H2S from liquid and/or supercritical CO2 that may be accomplished by adding an effective amount of a chemical scavenging agent to the carbon dioxide.
Another object of the invention is to provide a scavenging agent for removing H2S from liquid and/or supercritical carbon dioxide where the scavenging agent is easily made.
In carrying out these and other objects of the invention, there is provided, in one form, a method for scavenging a sulfur compound such as H2S and/or a mercaptan from carbon dioxide which can either be liquid CO2, supercritical CO2, and mixtures thereof. The method involves adding to the CO2 an effective amount of a triazine to scavenge at least a portion of the H2S.
It has been discovered that triazines and particularly hexahydrotriazines can effectively scavenge H2S from liquid and/or supercritical CO2 itself. The invention was discovered while resolving the problem of handing the additional volume of CO2 in a plant by removing the H2S corresponding to the increased production. While triazines are widely used in the natural gas industry for removal of H2S, and also used in removing H2S in tail gas processing where the tail gas comprises gaseous CO2, the application of non-regenerable chemical liquid scavengers such as triazines has not been known or documented for the removal of H2S from super-critical and liquid carbon dioxide prior to this invention. This is a very specialized application due to the fact that under supercritical conditions, CO2 exists as a single fluid phase.
The work done on the inventive method investigated the actual removal of H2S from supercritical CO2 at high partial pressures of CO2.
While many triazines are expected to be useful in the method of this invention, suitable hexahydrotriazines may have the formula: 
where R1, R2 and R3 are independently selected from the group consisting of straight or branched alkyl groups, substituted or unsubstituted with hydroxyl groups, where the alkyl groups average from about 1 to about 20 carbon atoms, preferably from about 1 to about 5 carbon atoms. Triazines, such as the preferred hexahydrotriazines of the invention, are readily made by reacting formaldehyde with a primary amine or an alkanolamine where the amine is primary. Suitable reactants with formaldehyde include amines of the formula H2NRX where RX can be defined as R1, R2, and R3 above. In one non-limiting embodiment, the preferred hexahydrotriazines include, but are not necessarily limited to, 1,3,5-tri-(2-hydroxyethyl)-hexahydro-S-triazine; 1,3,5-trimethylhexahydro-1,3,5-triazine; or mixtures thereof.
It is not always possible to predict with precision what the effective proportions of triazine are in removing or scavenging at least a portion of the H2S from supercritical or liquid CO2 since such dosage is typically dependent upon a number of complex, interrelated factors including, but not necessarily limited to, the partial pressure of the CO2, the temperature of the CO2, the particular nature of the triazine or blend of triazines if a blend is used, and the like. Nevertheless, in an effort to illustrate what an effective amount range may be, an effective amount of triazine ranges from about 1 to about 100 parts per part sulfur compound in the carbon dioxide on a weight basis. Preferably, the effective amount of triazine ranges from about 3 to about 20 parts per part sulfur compound, most preferably from about 3 to about 15 parts per part sulfur compound. These triazine scavengers have been shown to remove 1-2 pounds of H2S per gallon of triazine (120-240 kg/m3) from supercritical/liquid CO2, in one non-limiting embodiment of the invention.
In another non-limiting embodiment of the invention, the amount of H2S in the liquid or supercritical CO2 is reduced to less than 100 ppm, preferably less than about 80 ppm.
In application, the invention is a process for removing H2S from supercritical CO2 in a flow line using a triazine-based H2S scavenger. A method such as the following may be used, in one non-limiting embodiment.
A suitable injection point is selected for the scavenger injection preferably as far upstream of the monitoring point as possible.
The scavenger is then pumped into the target flow line via a high-pressure, positive displacement pump, using an injection quill and a pulsation dampener.
In turbulent flow regime, the scavenger will be homogeneously distributed into the main body of the fluid, and such regime provides efficient mixing.
In the presence of efficient mixing and optimized injection set-up, triazine-based scavenger will react irreversibly with H2S and form liquid reaction products.
The H2S concentration in the CO2 upstream from the injection point could be measured to establish the injection rate. The continuous scavenger injection rate could be continuously adjusted using a conventional feedback loop control system either from the upstream monitoring point, or a downstream monitoring point, or both.
In general, the triazine scavengers are water-soluble and are delivered in water as a solvent. Any other suitable solvent may also be used. The inventive process is applicable for removal of H2S from liquid and/or supercritical CO2 under conditions of temperature and pressure where injection of the treatment chemical does not increase the water content above its water saturation limit. For example, at 1200 psig (8300 kPa) and 70xc2x0 F. (21xc2x0 C.), the saturation limit is 130 pounds of water per MMSCF of CO2 (2.1xc3x9710-3 kg/m3), in one non-limiting embodiment of the invention. The volume of scavenger that can be injected should not introduce water exceeding the saturation point. The presence of any free water can cause very high corrosion rates in mild steel pipelines.
The triazine scavengers of this invention can remove H2S from liquid and/or supercritical CO2 at a wide range of temperatures and pressures, however; in general, the efficiency of the triazine scavengers increases with increasing temperatures and pressures.
As noted, carbon dioxide injection is widely used for secondary and tertiary recovery of hydrocarbons. Removal of H2S from liquid and/or supercritical CO2 will provide a cost effective treatment method to increase the volume of CO2 processed and used for injection into the formation that will yield improved recovery of the hydrocarbons.
The invention will be further illustrated with respect to the following Examples, which are not intended to limit the invention in any way but rather are included to give a better sense of the invention.